depolarisation of light , &c. 
43 
of depolarised light diminishes gradually with the tempera- 
ture. 
56. Rupert's drops of unannealed glass. When drops of 
melted glass are suddenly cooled by immersion in cold water, 
they acquire the faculty of depolarising light in every position, 
and of exhibiting the coloured rings by polarised light. At a 
certain thickness of the tail of the drop, the neutral axes are 
perfectly developed, and are parallel and perpendicular to the 
axis of the tail.* 
57. The semi-transparent and flat extremity of one of the legs of 
a young partan gives only a nebulous image of a candle, but 
depolarises the nebulous light in every position. 
58. A tubular film from the body of a partan, depolarises 
a small quantity of nebulous light. 
The following table contains a list of substances chiefly of 
animal and vegetable origin, which have no effect in de- 
polarising light. 
Gold leaf. 
Some crystals of diamond. 
Muriate of soda. 
Fluor spar. 
A crystal of spinelle ruby. 
Muriate of ammonia. 
Rochelle salts dissolved, and crystallized 
on the side of a glass. 
Nitrate of lead dissolved, and crystallized 
on the side of a glass. 
Sclerotic coat of a fish. 
Crystalline lens of a fish. 
Crystalline lens of a cow. 
Capsule of the crystalline lens of a fish. 
Ambergris melted and cooled. 
Film which surrounds the hydatids. 
Delicate film which lines the ribs of a lamb. 
Film from the stalk of the rhubarb. 
Film, or epidermis, which covers the shell 
solen ensis. 
* A full account of the properties of heated and unannealed glass will be found in 
two former papers. (Phil. Trans, mdcccxiv. Part II. p. 436, and mdcccxv. Part I. 
p.i.) 
